Force on a current currying wire in a magnetic field

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Homework Help Overview

The discussion revolves around a multiple-choice question related to the force experienced by a current-carrying wire in a magnetic field, specifically focusing on the direction of the force as indicated by a scale reading.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants explore the implications of the scale's positive reading and its relationship to the forces acting on the wire and the yoke. Questions arise about the attachment of the wire and the application of Newton's third law in this context.

Discussion Status

Participants are actively questioning the assumptions regarding the direction of the force on the wire and how it relates to the forces on the yoke. Some guidance has been offered regarding Newton's third law, but there remains uncertainty about the interplay between the current, magnetic field, and the forces involved.

Contextual Notes

There is a lack of clarity regarding the setup of the experiment, particularly concerning the attachment of the wire and the clamps, which may influence the interpretation of the forces at play.

maxelcat
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Homework Statement


This is a multi choice question from last years aqa a level. We know a positive reading is produced on the scales indicating something is pushing down on the scales.
Please see the picture.
stuck.jpg


Homework Equations


We know current direction, not B field.

The Attempt at a Solution


The positive reading on the scales suggests that the force on the wire is vertically down. Mark scheme says vertically up. We think Newton III is needed but not sure how it works.

Can anyone help explain why i is vertically up.
 

Attachments

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maxelcat said:
The positive reading on the scales suggests that the force on the wire is vertically down.
Is the wire attached to the scales?
 
nope. = the wire is not attached to the scales, though it is clamped at points D and E.
 
maxelcat said:
nope. = the wire is not attached to the scales, though it is clamped at points D and E.
Exactly. The yoke is what is attached to the scales. So rethink things with that in mind.
 
so the yoke is pushed down. Which means the wire is pushing it down. So there must be a downward force on the wire...
 
maxelcat said:
so the yoke is pushed down.
Yes.

maxelcat said:
Which means the wire is pushing it down.
Yes.

maxelcat said:
So there must be a downward force on the wire...
No. Consider Newton's 3rd law.
 
well I think you are getting me to say that if the wire is pushing the yoke down then the yoke is pushing it up... but that doesn't make sense to me because I can't see how the interplay between the current and the mag field works. Suppose I then took away the clamps that are holding the wire DE in place and changed nothing else. The wire would move upwards - yes?
 
maxelcat said:
well I think you are getting me to say that if the wire is pushing the yoke down then the yoke is pushing it up...
Sure. Newton's 3rd law in action. If A pushes B up, then B must push A down.

maxelcat said:
Suppose I then took away the clamps that are holding the wire DE in place and changed nothing else. The wire would move upwards - yes?
Yes.
 

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